Method of relaying by current transformer and relays



July 25, 1933. T. G. LE CLAIR 1,919,230

METHOD OF RELAYING BY CURRENT TRANSFORMER AND RELAYS Filed July 13, 1929 8 Sheets-Sheet 1 Li V V: Inuan 0r'- TiTfi' s ELEEIair' iml J Q MM/ @MALJWM/fl July 25, 1933. T. G. LE CLAIR METHOD OF RELAYING BY CURRENT TRANSFORMER AND RELAYS $heet 2 JAVAV 8 Sheets- .6/"'Q 633 Flled July 13, 1929 July 25, 1933. T. (5. LE CLAIR METHOD OF RELAYING BY CURRENT TRANSFORMER AND RELAYS $heet s 711 Filed July 15, 1929 10; 'A

Inuan arry 1933- T. G. LE cum. 1,919,230

METHOD OF RELAYING BY CURRENT TRANSFORMER AND RELAYS --Im" TiTE- ELEEIaiI" July 25, 1933. T. G. LE CLAIR 1,919,230

METHOD OF RELAYING BY CURRENT TRANSFORMER AND RELAYS Filed July 15, 1929 8 Sheets-Sheet 5 LEGEND: & CHARGING CURRENT RESISTANCE CURRENT -Im ren or TiTTs JlLaEZair i July 25, 1933. T. G. LE CLAIR 1,919,230

METHOD OF RELAYING BY CURRENT TRANSFORMER AND RELAYS Filed July 13, 1929 8 Sheets-Sheet 6 Inusnlu Tflus E2 LE Blair L H amw h/s $0M? HIT- s July 25, 1933. T. G, 1.1-: CLAIR METHOD OF RELAYING BY CURRENT TRANSFORMER AND RELAYS Filed July 13, 1929 8 Sheets-Sheet 7 Im rE1-L ur' 1115 E. LeEZan" MA m EM HUT July 25, 1933. T. G. LE CLAIR 1,919,230

METHOD OF RELAYING BY CURRENT TRANSFORMER AN] RELAYS Filed July 13, 1929 8 Sheets-Sheet 8 Patented July 25, 1933 PATENT OFFICE TIT'US G. LE CLAIR, OF CHICAGO, ILLINOIS METHOD OF RELAYING BY CURRENT TRANSFORMER AND RELAYS Application filed July 13, 1929. Serial No. 378,040.

This invention relates to a method of relaying by current transformers and relays and more particularly to such a method in connection with an alternating current line forprotecting the line against faults arising from short circuits between two or more of the line conductors and/ or between any one or more of the line conductors and ground. Long power' llnes generally comprise a number of series connected sections with suitable switching means connecting the sections.

In case ofa'fault on any of the sections power may be fed into the fault from either end of the'section or from both ends thereof depending upon the circuit employed in the particular system involved. Means of various kinds are used for disconnecting a sectionof a line when a faultoccurs, as indicated by an excess current flow into the section. It is one of the objects of the present invention to provide protecting means against such faults, the means being so arranged that a large fault current flowing to a fault on one section of the line will not bring about the disconnection of the intermediate sections through which it flows. The arrangement is such that I measure the current flowing into each section of the line and the c' rrent flowing out of each section of the line. If the current flowing out of the section at one end is substantially the same as the current flowing into the other end it is known that there is no fault ontha-t particular section and the means for disconnecting the section are not operated regardless of the magnitude of the current flow. If, however, there is -anappreciable difference between the amount of current flowing into a section at one end and the amount. flowing 'out of it at the other end then there must be a fault between the ends of the section and the means for disconnecting the section of the line from the rest of the line are operated.

In order to make the protective system ap .plicable alilre'to lines or sections of a line wherein power would be fed from both ends in case oi a fault as well as from one end only, my arrangement is. such that the functioning thereof is determined not only by the amount of current flowing at both ends of the section but also by the direction of flow at the two ends. If the direction of flow is such as to indicate that the current is flowing from one end of the section through the other the apparatus does not operate to disconnect the section of the line, while if the direction of how is such as to indicate a simultaneous flow of current into the section from both ends thereof then the apparatus operates to isolate the section. The protective system is not deiendent in its operation upon the establishment of an unbalanced condition of the line by the fault; hence, in a three phase system, for example, my protective apparatus will be effective on a three phase fault, where no unbalance is produced, as well as upon the occurrence of a fault between two of the phase conductors or upon the occurrence of a single phase to ground fault.

In a three phase system such as is here illustrated I provide, at one end of the section being protected, three pilot transformers each. of which is fed from current transformers on the respective line conductors. Each of the transformers has two series connected primary windings of identical electrical characteristics and a current of an amount controlled by the line current transformers is fed to the midpoint of the two primary windings. A relay energized from the secondary winding controls the opening of the line circuit. The transformers are connected in i, a resistor being interposed between each of the pilot windings and the neutral. pilot cable connects each of the pilot transformer primaries to corresponding windings on a similar set of transformers similarly connected at the other end of the section being protected. The resistors between the respective pilot transformers and the neutral are made equal to one half of the resistance of one of the pilot conductors. By this arrangement an electrically symmetrical pilot system is obtained so that there is a proper current flow in the respective transformer 9 windings under the various conditions to cause the line section to be opened or not to be opened as the case may require.

As previously stated, relays or other suitable means are controlled from the secondaries of each of the pilot transformers and control the opening of the line circuit. These relays operate when there is a given current flowing in either one of the two pricircuit with the primaries of the pilot trans-- formers to balance the circuit so that under condition, of through fault, that is a fault in a section beyond the section under considerati on the relays will not tend to trip due to the charging currentin the pilot cable. By the addition of this capacity the protection may be applied to considerably longer lines than was heretofore possible. I

One of the features of the present invention lies in the fact that practically the entire apparatus used is of standard construction. The only special apparatus required is the pilot transformers with the two primary windings and this transformer is one of the features of the invention.

The attainment of the above and further objects of the present invention will be apparent from the following specification, taken in conjunction with the accompanying drawings forming a part thereof.

In the drawings:

Figure 1 is a diagrammatic circuit drawing of my three phase pilot wire relay protective scheme;

Figure 1A is a fragmentary view showing the circuit connections between the tripping coil of the circuit breaker and the contact for controlling the same;

Figure 1B is a similar circuit diagram showing the connections in a system wherein separate circuit breakers are used for the different line conductors;

Figures 2 to 10 inclusive are diagrams of the circuit shown in Figure 1, with certain of the parts omitted for the sake of clearness, said figures showing the'path of current flow during the occurrence of various kinds of faults. In those figures:

Figure 2 shows the path of current flow during a phase to ground through fault;

Figure 3 shows the path of current flow during a phase to phase through fault;

Figure 4 shows the path of current flow during a three phase through fault;

Figure 5 shows the path of current flow due to ,a phase to ground fault fed equally from both ends of the power circuit;

Figure 6 shows the path of current flow during a phase to phase fault fed equally from both ends of the power circuit;

Figure .7 shows the path of current flow due to a three phase fault fed equally from both ends of the power circuit;

Figure 8 shows the path of current fiow due to a phase to ground fault fed only from one end of the power circuit;

Figure 9 shows the path of current flow due to a phase to phase fault fed from only one end of the power circuit;

Figure 10 shows the path of the current flow due to a three phase fault fed from only one end of the power circuit;

Figures 11, 12 and 13 show the effect of the current flow due to the capacity between the pilot conductors and the manner in which the condensers provided for the purpose neutralize the effect of this. capacity during a through fault. In those figures:

Figure 11 shows the current flow due to a phase to ground through fault;

Figure 12 shows the current flow due to a phase to phase through fault;

Figure 13 shows the current flow due to a three phase through fault; and

Figure 14 shows a modified form of my invention.

In the following description the current flow assumed is that for subtractive polarity current transformers, that is, on the diagrams if an instantaneous current is flowing into the current transformer primary from one end, theinstantaneous current is leaving the secondary at the same end.

Reference may now be had to Figure 1 for a more'complete description of the present invention.

The line to be protected is indicated at 1 and while I have herein shown a three phase power line, it is to be understood that the invention is not limited to any particular number of phases. The three conductors of the line are indicated at 2, 3 and 4 respectively. At one end of the line indicated at A, three current transformers indicated by the reference numerals 5, 6 and 7, are mounted upon the respective conductors 2, 3 and 4. One end of each of the current transformers 5, 6 and 7 is grounded and the other end extends through the usual measuring instruments 8, 9 and 10 to ground. The conductor from the current transformer 5 passes through a current transformer 11, beyond which it is grounded. Similar current transformers 12 and 13 are provided for the conductors from the current transformers 6 and 7 respectively. The upper terminal 15 of the current transformer 11 is connected to the midpoint 16 of the primary winding of a pilot transformer 17. The point 16 is at the exact midpoint of the primary winding 18, it being so located that the resistance, reactance, and impedance of the portion of the primary winding 18 between the point 16 and the end thereof 19 is exactly the same as that between the point 16 and the opposite end 20 of the primary winding.

and

' be more fully described. A resistance Electrically the portion of the primary winding between'16 and 19, which will hereinafter be referred to as the upper winding, is identical to the portion between 16 and'20, which will hereinafter be referred to as the lower winding, and it produces identical effects upon the secondary 21 of the transformer. A relay 22 is energized from the secondary 21 of the transformer and controls aset of contacts 23. When there is a current flow of a given value in either the upper winding 161 9 orin the lower winding 1(320, the secondary 21 is effective to cause the relay 22 to operate its contacts 23.

Half of the given value of current flowin through both of the primary windings win produce the same effect. Equal currents flowing in opposite directions in both of the primary windings will. neutralize one another the total magnetic effect upon the secondary 2 1 of the transformer will be ere. Hence, the relay 22 willnot be energized.

A transformer27 identical in construction with the transformer 17 is energized from the current transformer 12, which has a current flowing therethrough proportionate to the current flowing through the line conductor3 of the power line 1. A relay'32 is energized from the secondary 31 of the transformer 27 and controls a set of contacts 33 in the same manner that the relay 22 controls the contacts 23. In a like manner, a trans former 37 identical with the transformers 17 and 27 is energized from the current transformer13, which has a current flowing therethrough proportionate to the current flowing through the line conductor l of the power line 1'.

The transformer 37 controls a relay which controls a set of contacts 43 all in the manner previously set forth in connection with the transformer 17. The electrical connections to the three transformers 17, 27, and

37 are identical. One of the primary windings of the transformers 17, 27 and 37 are connected in Y, resistancesbeing inserted between the respective primary windings and the common point. To this effect a resistance 24, which is shunted by a condenser i vided for a purpose to be presently more. f set forth, connects the end 20 of the primary winding of the transformer 17 to the neutral conductor 26. 1

In alike manner the end of the primary winding of the transformer 27 is connected through a resistance 3st shunted by a condenser to the same conductor 26 and the end of the transformer 37 is connected through a resistance l l shunted by a condenser sl5 to-the same conductor 26.

The three current transformers 11, .12 and 13 are Y-connected being connected to the common conductor which is connected to the neutral conductor of the pilot system to 36 lfinergization of the coil shunted by a condenser 46 connects the conductor 26 with the conductor 50 and with the neutral pilot conductor.

At the opposite end of the power line 1, indicated in general by the reference numeral B, there is provided a set of equipment iden tical to the equipment provided at the end indicated at A. The apparatus at the end B is identical to the apparatus at the end A and the electrical connections are likewise identical. To this effect similar reference numerals have been used at the end B to indicate parts corresponding to the like parts at the end A, the reference numerals at the end B being primed.

Circuit breaker means is provided at the ends A and B of the line 1 for disconnecting the line from the rest of the system. This means at each end of the line may comprise a single breaker for simultaneously opening all three of the line conductors, or three sep rate circuit breakers may be used. In Fig ure 1, I show a three pole circuit breaker U at one end of the line, it being understood that a similar circuit breaker is provided at the opposite end of the line. The circuit breaker is provided v I the usual tripping coil 'i which is controlled by the relay coutacts 23, 33, and 13 in well known manner. In Figure 111, I show the circuit connections between the tripping coil and the controlling contacts 23, 33, and The three contacts are connected in parallel. with one another so that upon the closure of any one set of contacts a circuit is completed for the operating coil T, said circuit extending from the posi-' tive side ofa source of potential, through whichever one of the contacts 23, 33, or 43 that is closed, thence through the coil T, to the negative side of the source of potential.

T will cause tripping of the circuit breaker C. v

In Figure 1B, I show the circuit connections for using three separate breakers (1;, C and Q for openingthe line conductors 2, 3, and a respectively. A similar arrangement is provided at the end B of the line 1,. In this arrangen'ient it is apparent that the breaker C will. trip upon closure of the set of contacts 23, the breaker C will trip upon closure of the set of contacts and the breaker C. will trip upon closure of the set of contacts 43. The circuit in each case e tends from the positive side of a source of potential, through the corresponding set of contacts and the corresponding trippag coil, to the negative side of the source of potential.

A pilot cable indicated at and comprising the conductors 5G, 57, 58 and 59 connects the apparatus at A with the apparatus at B. The conductor 56 connects the point 19 of the pilot transformer 17 with the point 19 of the pilot transformer 17'; the conductor 57 con nects the point 29 of the primary winding of the transformer 27 with the point 29 of the primary winding of the transformer 27'; and the conductor 58 connects the point 39' of the primary winding of the transformer 37 with the point 39 of the primary winding of the transformer 37. The conductor 59' comprises the neutral of the pilot cable and is connected at one end to the conductor 50 and at the other end to the conductor 50.

The resistors 2- 34, 44, and 36 are all of identical value being of half the resistance of one of the pilot wires 56 to 59 inclusive. The resistors 2d, 34, 44 and 36 are of the same value as are the resistors at the end A being likewise of half the value of the resistance of one of the pilot conductors 56 to 59 inclusive. The condensers 25, 35, 4-5, and 46 are all of identical value being of one fourth of the capacity to neutral of one of the pilot wires 56 to The corresponding condensers 25, 35, 45 and 46 are of a like value.

The current transformers 5, 6 and T and the transformers 5, 6 and 7 are the usual line current transformers having a ratio such that the currents in their secondary circuits is of the value desired for the instruments and other relays employed. The current transformers 11, 12 and 13 and the transformers 11, 12 and 13 have a ratio dependent upon the conditions under which the pilot wire circuits and relays are to operate. Their purpose is to insulate the pilot w' 'e circuit from ground and to reduce the current in this circuit to a value low enough that the voltage drop in. the wires will not be excessive. The condensers 25, 4:5,35, and 46, as

well as the condensers 25, 35, 4:5 and d5 are provided to prevent an unbalance in the system being produced by a through ground due to the charging current of the pilot wires. These condensers compensate for the charging current in a manner to be more fully eX- plained as the description proceeds.

There are three general types of faults to be considered in. connection with the protective apparatus herein shown. One of these faults is what may be termed a through fault, that is a fault external to the section between the points A and B with the fault current timving through the section; the second type of fault here to be considered is a fault between the points A and B with the power fed equally from both ends of the circuit; the third type of fault to be considered is a fault between the points A and B with power fed from only one end of the power circuit. In either of the three major causes of faults the fault may be from one phase to ground, or between two of the phases, or between all three phases, making a total of 9 different kinds of faults to be here considered.

An explanation will now be given of the behavior of the system under various kinds of faults and for this purpose reference may be had to Figures 2 to inclusive showing the path of current flow in the pilot circuit under the various conditions. In these circuits the secondaries of the pilot transformers have been omitted as well as the relays controlled by the secondaries. Likewise the condensers 25, 35, 45, 46, and 46 have been omitted for the sake of clearness.

In the following description the eflects of these condensers will be entirely ignored and an explanation of the effects of the condensers will be subsequently given as the description proceeds.

Reference may now be had more particularly to Figure 2 showing the path of current flow through the pilot circuit due to a phase to ground through fault. The phase conductor 2 is shown to be grounded at and the direction of power flow in the line is as shown by the arrow 61. At. a given instant when the direction of cur rent flow in the line conductor 2 is as indicated by the arrow 61 the direction of flow of current in the current transformer 5 will be 180 cut of phase therewith or in a direction as indicated by the arrow (32. The direction of flow of current through the current transformer 11 will be 180 out of phase with the current flowing through the current transformer 5 hence will be in a direction such as indicated by the arrow 63. In a like manner the direction of current flow through the transformer 5 will be as indicated by the arrow (52 and the direction of current flow through the transformer 11 will be as indicated by the arrow 63. Assume that the value of the current flowing through the transformer 11 and through the transformer 11 I amperes.

At, 16 the current from the transformer 11 divides over two branches, one of which includes the lower half of the pilot transformer primary winding 17 and the other of which includes the upper half of the primary winding of the transformer 17'. The portion of the current flowing through the lower half of the primary winding of the transformer 17 extends through the resistor 24-, the resistor 36, thence by way of the conductor 50 to the other end of. the currenttransformer 11 whereas the current flowing through the upper half of the primary winding of the transformer 17 extends through that winding thence by way of the pilot conductor 56 and the upper half of the primary winding of the pilot transformer 17, through the w' iding 1.1 of the current transformer thence by way of the conductor- 50 through the pilot line 59 to the conductor 50 and the lower terminal of the current transformer 11.. It is to be noted, that the first mentioned path includes only two of the resistors whereas the last mentioned path includes none of the resistors and instead two of the pilot conductors. The resistance of the two pilot conductors is just twice that of the two resistors 24? and 36. It is, however, to be noted that the path through the upper half of the primary winding of the pilot transformer 17 includes the current transformer 11 through which the current flow is in phase with the current flow through the current transformer 11.

Hence, although this latter path has twice the resistance of the forn'ier path, there is twice the potential forcing the currentthrough that path. ltmay, tl1erefore,he seen that half of the I amperes of the current transformer 11 flow through the lower half of the primary 'inding of the transformer 17 and one half I amperes flow through the upper half of that transformer windin The value of, the current flowing over the path indicated by the arrow 65 is, therefore, one half I amperes and the value of the current flowing over the path indicated by the arrow 66' is also one half I amperes. In a likemanner there being I amperes flowing; through the current transformer 11, the direction of flow being as indicated by the arrow 63 it may be seen that one half I amperes flow through the upper portion of the primary winding of the transformer 17 as indicated by the arrow 65 and, therefore, one half I amperes flow through the lower half of the primary winding of this transformer since the current flow induced by the transformer 11 and 11 are in phase. The direction of flow through the lower half of the primary winding of the pilot transformer 17 is, therefore, one half I amperes as indicated by the ar row (36. 7

Since the currentiiow in the upper and in the lower half of the primary windings of the two pilot transformers l1 and 17 are in; opposite directions and of equal values they neutralize one another with the result that no voltage is induced in the secondary windings of these transformers. T he relays controlled from the secondary windings of these two transformers are, therefore, not energized. 7 It may be readily shown that under the conditions contemplated there is no current ii wing through the primary windings of e transformers 27, 37, 27' or 37'. This so because the conductor 26 which at one end of the primary windings of the transformers 2? and 37 is at electrically the same ixotential as is the conductor 26' at the other end of a eircuit for any current that would have to flow through the primary windings of those tTfIDSfOI'lIHIS. No current can he forccdfrom the conductor 26 or from the conductor 26' through the windings of the current transforinersfi, 13, 12 or i because of the relatively high impedance of those transformers. It may thus be seen that, ignoring for a-moment any eflect that charging current between the pilot conductors may have, there is no possibility of any of the relays 22-, 32, i2, 22', 32 or l2 operating um 1 condition of a through phase to ground is ited out later the charging current 1liZGCi by the our cat furnished by t 1e condensers 25, 35, 45, 46, a5 and 5 6 operated by the ch iii. xvlanat e 1 math of current an currents. in non he given of the ow due to an interphase i 'ougn fault. Under these conditions the a. z. min w-m. 3 1 1. iii in is as lla'unlbithvc in figure anc be had to that figure for a understanding thereof. The

fault is as.

nod to be at the point 70 and ween the line conductors 2 and 3.

he" The current iiow at iron instant may he asicated by the arrow 71 2) and by the arrow 79 3. T 1e current flow igh the current transformers 5 and 6 indicated. by the rows 73 and 74;

iy and, therefore, the current flowransforniers 11 and i r the arrows 75 and Th e current flowing through cii'ner. 5 and 6' will, thereicateil by arrows 73' and current flowing through the s ll and 12 will be as indicated or 1 arrows T5 and 1 6. i

Assume that a. current of I amperes flows through the current transformers 11 and 12 respectively and that a like current flows through the transformers l1 and 12 respectively. That the current flowing through each of these current transformers will be of the same value is evident from the fact that the inducing cause for the current flow ing through those transformers is the same, namely the current flowing through the conductors 2 and 3, and likewise the circuits for each of the transformers 11, 1 2, 11 and 12 are identical. From the values of the resistances and impedances given it may be shown that the current flowing through the closed loop comprising the transformer 11, the lower half of the primary winding of the pilot transformer 17, the resistor 2a, the portion of the conductor 26, the resistor 34, the lower half of the primary winding of the pilot transformer 27, the current transformer 12 and the portion of the conductor between the current transformer 12 and the current transformer 11 is the same as the current flowing through the closed loop comprising the upper halves of the primary windings of the four pilot transformers 17, 27, 17 and 27 as well as the two pilot conductors 56 and 57 and the four current transformers 11, 12, 11 and 12. Since the current flowing through both of these mentioned loops is the same, it therefore, follows that ifithere are so that none of the relays can be I amperes flowing through the transformer 11' there will be one half I amperes flowing through the upper half of the primary winding of the transformer 17 and likewise one half I amperes flowing through the lower half of the primary winding of the transformer 17 The direction of flow of current through the primary windings of-the pilot transformers 17, 27, 27 and 17 is as indicated by the arrows, the current flowing in the two halves of each of the primary windings being in opposite directions and of equal amounts. As a result the current through the two windings of each of the pilot transformers neutralize one another with the result that the secondary windings of those transformers is not energized, hence the relays controlled by the secondary windings is not energized. In a manner similar to what was shown above in connection with the pilot transformers 27 and 37 in case of a through fault to ground it may be here shown that there is no current flowing through the primary windings of the pilot transformers 37 and 37 due to an interphase through fault such as here contemplated. It may thus be seen that under the second of the three possible types of through faults the relays controlled by the pilot transformers do not operate.

It will now be shown that the relays controlled by the pilot transformers do not operate upon the occurrence of a three phase through fault which is the third type of through fault to be considered and for this purpose reference may be had more particularly to Figure 4. 1

The fault is assumed to be locatedv at'the point and the direction of power flow is as before assumed to'be from B to A. The fault is assumed to include all three-of the line conductors hence there will be current flowing in all of the line conductors. Assume a current flow through the conductors 2. 3 and 4. as indicated by the arrows 81, 82 and 83. The current flow through the current transformers 5, 6 and 7 and through the transformers 5, 6 and 7 will. be 180 out of phase with the current flowing the conduc tors 2, 3 and 4 respectively and will be in a direction as indicated by the arrows above each of those current transformers. The current flowing through the transformers 1.1, 12 and 13 and the transformers 11. 12' and 13' under the conditions assumed will be respectively as indicated by the arrows. While. a matter of fact. the amount of current flowing through each of the transformers 11, 12 and. 13 and the transformers 11, 12 and 13 will be identical still this is not essential to the operation of the system and, in the explanation of the operation to be presently given. no assumptions will be made as to the equality of the amount of current flowing through those transformers.

Assume that there are I amperes flowing through the transformer 11, that there are 0 I amperes flowing through the current transformer 12 and that there are 0 I amperes flowing through the transformer 13 where c and 0, may be constants whatsoever. The current flowing through the transformers 11, 12 and 13 will necessarily be the same as the currents flowing through the transformers 11, 12 and 13 respectively since the inducing currents are, in each case, the currents flowing through the line conductors 2, 3 and at. By a process of reasoning similar to that employed above in connection with the explanation given of the division of current in the circuit shown in Figure 2, it may be shown that the I amperes flowing through the current transformer 11 divides in two at the primary windings of the pilot transformer 1'7 so that half the current flows through. the upper half of the winding and one half of the current flows through the lower half of the winding. The current flow indicated by the arrow 85' is, therefore, one half I amperes and that indicated by the arrow 86 is likewise one half I amperes. In a. like manner if the current flowing through the transformer 12 is c I amperes then the current flowing through the upper half of the primary winding of the pilot transformer 27 will be as indicated by the arrow 87 and will be one half 0 I amperes and a like amount of current will flow through the lower hal f of that primary winding in a direction such as indicated by the arrow 88'.

Similarly the 0 I amperes, flowing through the current transformer 13 will divide through the windings of the transformer 37' so that one half o I amperes will flow through the upper windings in a direction such as indicated by the arrow 89 and a like current will flow in the opposite direction through the lower windings of the primary of that transforn'ier as indicated by the arrow 90. The current flowing through the upper halves of the primary windings of the pilot transformers 17, 27 and 37 will be as indiated by the arrows 85, 87 and 8.) and will be in amounts equal to that through 85', 87' and 89' respectively. In a like manner the currents indicated at 86, 88 and 90 will be equal respectively to the currents indicated at 85, 87 and 89 respectively and in opposite directions. If the vector sum of the current flowing through the current transformers 12 and 13 is equal and opposite to that flowing through the transformer 11; that is if the values of 0 and 0 used above, are unity, then there is no current flowing through the neutral pilot conductor 59; whereas if the currents are not of equal values then there will be a current flowing through this conductor said current being of value equal to the vector sum of the currents indicated at 86, 88

the current flows being of the same va tlzrough the pilot conductor 56 nor the upper halves of the primary windings These currents neutralize one another in so far as effect upon secondaries of the respective transformers are concerned; hence, regardless of the magnitude of the current flow, the relays controlled by the secondaries of these transformers are not energized.

An explanation will now be given showing that upon the occurrence of a fault between the ends A and B the current flowing through the pilot circuit will be such as to cause the energization of the relays 22, 32, i2, 22, 32 and 42 as the case may be. Reference may be had to Figures 5, 6 and 7 showing the paths of current flow due to a phase to ground fault, a phase to phase fault, and a three phase fault respectively in a system wherein faults are fed equally from both ends of thepower circuit.

'Reference may be had now to Figure 5, wherein aphase to ground fault is assumed to be located at the point 100. Current is fed from the end A of the line conductor 2 to the ground 100 as indicated by the arrow 101 and at the same time current is fed from the end B of the line conductor 2 to the ground fault as indicated by the arrow 101.

At any given instant these currents may be assumed to be flowing in a direction such as indicated by the arrows and, since it is assumed that the fault is fed equally from both ends of the power circuit, these currents will be of equal amounts. The current induced in the transformer will, tl'ier-efore, bein a direction such as indicated by the arrow 102 and the current c ed in the transformer 5 will be in a direction s as indicated by the arrow 102. The current flowing through the transformer 11 will be in a direction such indicated by 103 and that flowing through the transformer 11 will be in a direction such as indicated by 103.

no i

both cases. Hence, at any given instant,

electrical potential of the point 16 the same as that of the point 16 and there no current flowing between these two points, that is, there will be no current flowing; through of th pilot transformers 17 and 17. The current froin the transformer 11 will flow through the lower half of the primary winding of the transformer 17, thence by way of the resistor 24: to the conductor 26, thence by way of the resistor 36 to the conductor 50 tnree pole c and back to the other terminal of the current transformer.

The current from the current transiornior 11 will flow through e lower half of the 7 primary windi of 1r lot 11''101111L1 1T, thence b'; I resistor l co;

conductor 50 ductor 26', resistor back to the other rim of the current I i v n n transformer ll. 'lhe 2e and at the seine elect 1 potential hence there can be no current flow betveen these conductors by way of the pilot ansformers 27 and 2? or 37 and 3? Th ie can no current flowing from the conductor 26 to the v cond rotor by way u l lower primary windi of the transfo 2 rent tl'illlS'i i'll'li primary winr "r 12 or .1 may of the lower s of the trzuisformer and the current tra ornicr 13 due high impedance oi rurrent transfer 12 and 12%. The a- :e m plies as to any eucy for the current to flow between the conrluctor through the he ductor 20' and the co, lower half of the pr transformer 1" or of tile Current flowing through the i0 or half of the primary winding the QYZlLcLOl'illOl' 17 nduces a potential in he secondary of that transformer with the result that the relay 22, which is connected across the secondary of that truosfoiuie is cne ing a circuit between its c 1 like manner currelt fun lower ha f of the p"""nary ui transformer int hues a Vital-Mal in 5h ondary winoiug of it transforumr resulting in the energization of the r-Liay 22 with tie result that the contacts 23 controlled thereby are closed.

The operation of the relay eauses the circuit breaker, or the like, at the end A of the line to be rip ed n the well known manner, and 7' i of the relay 22 causes the circu Ker or the hire at the end 5 of section e line to be imewise tripped. up he o huh?! L circuit breakers are used for each i no u 're operathrough the 1 tion of the relay must: the 'rippins s of the 1 ==z breaker; whereas, if av 'r used then the three re'ays 22. ng of the circuit operation VJ: e manner or open A to a phase to phase new be given of the i e sy tem in resp ms=.

(3. The fault is assu 4 point 110 betu'ee 'l to be m. sated at the V so conductors 2 and 3 and the curr-oi 1 you instant be indicated by @118 a; i and 112 which show the path of our nt lowing at the end A of line through the two conductors 2 and 3 by way of the fault; and the arrows ing 07: the

111 and. 112 which show the path of current flow at the end I3 due to the fault.

The direction of flow of current through the transformers 5 and 6 will be as indicated by the arrows 113 and 114 respectively and the direction of current flowing through the transformers 5 and 6 will be as indicated by the arrows 113 and 114 respectively. The resultant current flowing through the transformer 11 will be as indicated by the arrow 115 and that flowing through the fault to transformer 12 will be as indicated by the arrow 116, these currents being in opposite directions due to the fact that the current flow through the conductors 2 and 3 at the transformers 5 and 6 are in opposite direc tions. In a like manner the direction of current flow through the current transformers 11 and 12 will be as indicated by the arrows 115 and 116 respectively. The points 16 and 16 of the pilot transformers 17 and 17 are at the same electrical potential, hence there will be no current flowing between these points. As a result there is no current flowing through the upper half of the primary windings of the transformer 17 nor through the upper half of the primary windings of the transformer 17, nor through the pilot conductor 56. In a like manner the upper terminal of the current transformer 12 which corresponds to the mid point of the primary winding of the transformer 27 is at the same electrical potential as is the upper terminal of the current transformer 12 which corresponds to the potential of the mid point of the primary winding of the pilot transformer 27 and for this reason there will be no current flowing through these upper windings nor through the pilot conductor 57.

Consider the loop indicated in heavy lines in the figure and extending from the point 16 through the lower half of the primary winding of the transformer 17 the resistor 24, a portion. of the conductor 26, the resistor 34, the lower half of the primary winding of the transformer 27, the current transformer 12, a portion of the conductor 60, and the current transformer 11 back to the point 16. In this loop the direction of the arrows 116 and 115 are the same, hence there will be a current flowing through the loop. The current flowing through the lower half of the primary winding of the pilot transformer 17 will induce a voltage in the secondary of that transformer and likewise the current flowing through the lower half of the primary winding of the transformer 27 will induce a voltage in the secondary of that transformer. As a result the relays 22 and 32 are energized thereby closing the contacts 23 and '33; In

a like manner there will be a current flowing through the loop extending from the point 16 through the lower half of the primary winding of the pilot transformer 17 through the resistor 24, a portion of the conductor 26, the resistor 34;, the lower half of the primary winding of the pilot transformer 27', through the current transformer 12, a portion of the conductor 50 and back through the current transformer 11 to the point 16 thus completing the loop.

The current flow through the pilot transformer windings will result in an energization of the relays 22 and 32 connected across the secondaries of those transformers. It is to be noted that the conductors 26 and 26' are at the same electrical potential, hence there will be no flow of current between these conductors by way of the primary of the pilot transformer 37 and the pilot conductor 58 and the pilot transformer 37. It may likewise be readily shown that the conductor 26 is at the same electrical potential as is the conductor 50, hence there will be no current flowing through the resistor 36 nor will there be any tendency for current to flow through the lowerwinding of the transformer 37 and the transformer 13. The above likewise applies to the corresponding parts of the system at the ends marked B.

Reference may now be had to Figure 7 showing the circuit condition due to a three phase fault on a system fed equally from both ends of the power circuit. In this case the three phase fault is indicated at 120 and the direction of current flow at a given instant in the respective conductors 2, 3 and 4 of the line may be assumed as indicated at 121, 122, and 123 from the end A and as indicated at the 121, 122 and 123 from the end B of the section. The direction of current flow through the line current transformers 5, 6 and 7 is as indicated by the arrows with the result that the induced current in the transformers 11, 12 and 13 is as indicated by the arrows.

At the other end of the line indicated at B, the direction of current flow through the current transformers 5, 6' and 7' is as indicated by the arrows and the corresponding current flow through the transformers 11, 12 and 13 is likewise as indicated by the arrows. Assuming that the fault is fed equally from both ends of the line and since the apparatus associated with each one of the line conductors is identical to that associated with the others it may be seen that the potential at the upper end of each of the cur rent transformers 11, 12 and 13 which is the potential applied to the mid point of the respective pilot transformers 17, 27 and 37 is the same as the potential applied to the mid point of the corresponding pilot transformers 17, 27 and 37, hence there will be no current flowing from the midpoint of the corresponding transformers on the opposite ends of the line through the upper windings of those transformers and through the pilot conductors 56, 57 and 58. Current from the three current transformers 11, 12 and 13 1:5

flows through the corresponding lower halves of the pilot transformers 17 27 and 37 respectively to the neutral conductor 26. Due to the fact that there is a balanced condition in the pilot circuit there will be no current flowing through the neutral 59.

The path of current flow through the pilot transformers at the end B of the line is identical to that at the end A as above set forth, and is indicated in heavy lines in the figure. Under a fault such as is here contemplated the lower half of the primary winding of each of the pilot transformers is energized with the resulting energization of the secondary of the transformers and of the relays connected across the secondaries of the respective transformers. As a result, during the occurrence of a fault such as is here contemplate all of the relays energized and, through the usual instrumentalities, bring about the disconnection of both ends of the line from the rest of the system.

An explanation will now be given of the resultant current flow due to a phase to ground, a phase to phase, or three phase fault in a system wherein the power is fed from one end of the circuit only, and for this purpose reference may be had to Figures 8, 9 and 10. It will be shown that upon occurrence of a fault such as is here contemplated both ends of the section will be opened even though the section is being fed from only one end of the power circuit.

Reference may now be had more particularly to Figure 8 wherein the power is assumed to be fed to the section of the line only at the end B andthe phase conductor 2 is grounded at 130. The direction of the current flow in the line conductor 2 of the power line may be assumed to be as indicated by the arrow 131. In the current transformer 5' there is induced a current which flows in the direction indicated by the arrow 132, resulting in a current being induced in the current transformer 11. The direction of flow of current through the transformer 11 will be as indicated by the arrow 133. The current flows from the transformer 11 to the point 16 which is the mid point of the primary winding of the transformer 17, and at that point the current divides into two paths. One of the paths includes the lower half of the primary winding of the transformer 17, the resistor 24k, conductor 26, resistor 36, thence by way of conductor 50 to the opposite terminal of the current transformer 11. The second path of current flow from the point 16 is a follows; the upper half of tie pilot transformer 17 pilot conductor 56, to the point 19 of the primary winding of the pilot transformer 17, thence through the primary windings, the resistor 24, the resistor 36, conductor 59 to the conductor 50 and the other terminal of the current transformer 11.

In view of the fact that the resistors'24s, 36, 2a and 36 are all of equal value and of half the resistance of lines 56 or 58 it may be seen that the path of current flow including the upper half of the pilot transformer winding 1? includes t times as much resistance as is included in the path of current flow through the lower half cf the primary winding of the transformer 17". It is, therefore, evidentthat one third as much 0 rrent will flow through the upper half of the pri mary winding of the transformer 17 as will flow through the lower half of the winding. l'lence, assuming that I amperes flow through the current transformer 11 then one fourth l ampercs will iiow through the upper half of the primary of the transformer 17 and one quarter 1 amperes will flow through both the upper and through the lower half of the primary winding of the pilot transformer 17, whereas three quarters l amperes will flow through the lower half of the primary winding of the pilot transformer 17. It may be seen that the current flowing through the upper half of the primary winding 17 is in a direction opposite to that of the how of current in the lower half of that winding, hence the effect of these two windings will be differential.

Since the upper half of the winding carries one quarter I amperes and the lower half carries three quarters l amperes the net effect will be the same as though one half I amperes were carried by one only of the two windings. In the primary winding of the pilot transformer 17 the current is flowing in the same direction in the upper half as it is in the lower half each half carrying the one fourth I amperes, and the effect is the same as though one only of the windings were ener gized with one half I ampercs. t may readily be shown that the conductor 26 is at the same electrical potential as in the conductor 26, hence there will be no tendency for current to flow between these two conductors by way of the primary windings of the trans formers 27 and 37 and the pilot conductors 57 and 58 through the primary r-rinding of the pilot transformer 27 and 37. Practically no current will flow through the lower halves of the pilot transfor iers 27, 37, 2? and oi and the associated current transformers due to the rather high impedance of the current transformers. As a result the current flow through the pilot circuit-s will be substantially as shown in heavy lines in Figure 8. The relays controlled by he secondaries of the pilot transformers l7 and 17 will be energized and, at their contacts 23 and 23 respectively will establish a circuit to the various instrumentalities for disconnecting both ends of the conductor 2 from the circuit.

Reference may now be had to Figure 9 showing the current ii ow in the pilot circuit due to a phase to phase fault in a system wherein the fault is fed from only one end of the power circuit. The fault is assumed to be located at between the phase conductors 2 and 3. The fault current through the conductors 2 and 3 may be assumed to be in the. direction shown by the arrows 141 and 142 at a given instant. The resultant current flow through the current transformers 5 and (3 will be as indicated by the arrows 143 and 144 respectively, and the resultant current flow through the current transformers 11 and 12 will be as indicated by the arrows 145 and 146 respectively. The current flowing due to the voltage induced in the transformers 11 and 12 extends over two paths as follows: Path No. 1 extends from the upper terminal of the transformer 11, through the lower half of the winding of the pilot transformer 17 the resistor 24, a portion of the conductor 26, the resistor 34, the lower half of the primary winding of the pilot transformer 27, the transformer 12', a portion of the conductor 50, back through the current transformer 11 to the starting point.

The second path extends from the same starting point, the upper half of the primary winding of the pilot transformer 17, the pilot conductor 56, both halves of the primary winding of the pilot transformer 17, resistor 24, a portion of the conductor 26, resistor 34, both halves of the primary winding of the pilot transformer 27, pilot conductor 57, upper half of the primary winding of the pilot transformer 27, current transformer 12, a portion of the conductor 50', back to the starting point through the power transformer 11. From the valves previously given it may readily be seen that the impedances of the first path mentioned is one third that of the second mentioned path, hence there will be three times as much current flowing through the first path as will be flowing through the second path. Assuming that I amperes flows through the current transformers 11 and 12 it, therefore, follows that one fourth I amperes flows through the sec ond mentioned path and three fourths I am peres flows through the first mentioned path. Since the current through the upper half of the primary windings of the transformers 17 and 27' is in a direction opposite that of the current flow of the lower half of the primary windings of those transformers it, therefore, follows that the net effect will be differential and will be the same as would be produced by a current flow of one half I am peres flowing through one only of the two halves of each of the windings.

The current flowing through the two halves (if the primary windings of the pilot transformer 17 is in the same direction as is also the case with the current flowing through the two halves of the pilot transformer 27. As

the result the effects of the currents through the two windings of each of these transformers will be cumulative and although one fourth I amperes flows through each half of each of the transforn'iers the effect upon the transformers 27 and 17 is the same as would be produced by one half I amperes flowing through one only of the halves of the windings of each of these transformers. As a result the secondaries of these transformers are energized to the same extent as are the secondaries of the transformers 17 and 27' with the result that the relays 22 and 32 operate as well as do the relays 22 and 32. As a result of the operation of these relays the line conductors 2 and 3 are disconnected at both ends from the circuit. It may be shown that the conductor 26 is at the same electrical potential as is the conductor 26,

hence there will be no tendency for current i to flow through the primary windings of the pilot transformers 37 and 37 between the conductors 26 and 26. It may likewise readily be shown that the potentials of the conductors 26 and 50 are the same, hence there will be no tendency for current to flow between them or for currents to flow from the conductor 26 through the resistor 36 to the conductor 50.

Reference may now be had to Figure 10 showing the path of the current flow in the pilot circuit due to a three phase fault on a system wherein the current is fed to the fault from one end only of the power circuit. The fault between the conductors 2, 3 and 4 is indicated at 150 and the fault current flow in the respective conductors may be represented by the arrows 151, 152, and 153. The resultant flow of current through the transformers 5', 6 and 7 is as indicated by the arrows 1 above those transformers and the current flowing through the transformers 11., 12 and 13 is likewise as indicated by the arrows. Assume that I amperes flows through the current transformer 11, (r I amperes flows through the current transformer 12 and that b I amperes flows through the current transformer 13 where a and 7) may represent any constants. It may readily be seen from the discussion given above in connection with Figures 8 and 9 that one fourth of the current flowing through the transformer 11' flows through the upper half of the primary winding of the transformer 17 and three fourths of the current flows through the lower half of the winding. In a like manner one fourth of the current carried by the transformer 12 flows through the upper half of the primary winding 27 and three fourths of the current flows through the lower half of the winding, and one fourth of the current carried by the transfori'ner 13 flows through the upper half of the primary winding of the pilot transformer 37 and one fourth of the current flows through the lower half of the winding.

The currents flowing through the two windings of the transformers 17, 27 and 37 are, in each case, in opposition and the net effect is difierential. In the two halves of the primary windings of the tra sformers 17, 27, and 37 the current flows in the same direction hence the net effect is cumulative. The net effect of the differential action on the two windin s of the pilot transformers 17 is the same as would be produced by one half I ainperes flowing through only one of the two windings. The net effect of the cumulative action upon the two halves of the windings of the transformer 17 is also the same as would be produced by one half I amperes flowing through one only of the windings. In a like manner the net effect produced by the currents flowing through the two halves of the primary winding of the transformer 27 is the sameas would be produced by one half a I amperes flowing through one only of the windings and that produced upon the pilot transformer 37 is the same as would Z) T amperes through one only of the two windings. The same effects are produced in the transformers 27' and 37 respectively. As a result a voltage is produced in the secondary windings of all SlX of the pilot transformers and corresponding relays, 22, 22, 32 and l2 are energized and bring about the disconnection of both ends of the line conductors 2, 3 and 4|: from the power circuit.

From the above discussion in connection with Figures 2 to 10, inclusive, it may be seen that the action of the current flowing through the pilot transformers when eX pressed in terms of ampere turns is in each of the nine cases equal to a constant times the difference in the current values at the two ends of the respective conductors of the section under consideration. Under the conditions represented in Figures 2, 3 and f the currents at the two ends of each conductor are equal and in the same direction hence their difference is Zero and therefore they produce zero effect on the pilot transformers. Under the conditions illustrated in Figures 5, 6 and 7 the currents at the two ends of the section are assumed to be approximately equal and in opposite directions. Therefore the difference (vectorially) is twice the current at one end. Since a half of the resultant of this difference flows through half of the pilot transformer the net effect is the same as would result from one-fourth of this current difference flowing through the entire primary. In the instances illustrated in Figures 8, 9 and 10, there is current at only one end of the line, therefore the difference between the current at the two ends of the line is equal to this current flow. A current be produced by one half which is a function of one-fourth of the line current flows through both halves ofthe pilot transformers 17-21-37. This, therefore, produces the same effect as is produced under the conditions illustrated in Figures 5, 6 and 7 It has been previously shown that the differential effect of the current flowing through the two halves of the pilot transformers 172737 in the circuits shown in Figures 3, 9 and 10 is the same as that in the pilot transformer 172737 of those circuits. Thus it may be seen that regardless of the type of fault or the type of conditions prevailing, the net effect, insofar as concerns the pilot transformers, will be the same at both ends of the section and will always be the same function of the vector difference of the current at the two ends of the section under consideration. WVhen the vector difference of these currents is zero the net effect on the pilot transformers will be Zero. This is the condition that prevails when the current flows in the same direction at both ends of the section, that is, under normal conditions or under condition of through faults.

in the above discussion in connection with 2 to 10 inclusive a system was rWllGlEtlH the condensers 25, 35, 45, 46 and 25, 35, 45 and 46' were not necessary. On short lines wherein the capacity between the pilot wires is small these condensers may be omitted. Where long pilot wires are used their charging currentwould produce an unbalance and the conditions set forth above in connection with Figures 2 to 10, with the probability of the tripping the relays on through faults, unless compensating capacitors such as shown in Figure l are used.

he condensers, 25, 35, a5, 46, 2-5, 35, 45' and 46 are each of a capacity of one fourth of the capacity to neutral of one pilot wire.

t may be shown that as a result of the presence of these condensers the effect upon the pilot transformers of the charging current of the pilot cable is neutralized upon the occurrence of through fault. For this purpose reference may be had to Figures 11, 12 and 13 showing respectively the portions of the pilot circuits outlined in heavy lines on Figures 2, 3 and 4. The corresponding diagrams are the same except for the addition of the capacitors 25, 25, 36, 36.

Phase to ground and phase to phase through fault can be grouped together in that they give the same magnitude of current flow across the compensating resistors. For the three phase through fault a separate eX- planation will be given. Reference may now be had more particularly to the Figures 11 and 12 showing the circuit conditions for a phase to ground and phase to phase through fault.

The capacitors Ca, C C and Q; are each equal to one eighth the capacitance to neutral tion, Figure 13 shows how ofone pilot wire, and they are only a hypothetical representation of the distributed capacity of the pilot wire circuit so placed as to give close approximation of the actual operating conditions. The capacitors (1., and C are in effect one at each end of pilot wire circuit and C and C the mid-point. Suppose, for exampie, the resistors 24 and 36 each equals one ohm "nd I equals one ampere, then the petenti at the various points of the circuits will be as indicated on the diagrams. The pot 1. across O is one-half volt and the ch 4 current that it will supply to the poinof ero potential will be, let us say, ampere indicated on the diagram. The poten 1 across the condenser 25 is only one fou volt or one half that across C but the 1 user has coir e twice the capacitance of (1, it will supply the same amount of charging current, cancelling that from C The e process of reasoning can be applied to the other end of the circuit. Therefore, the balanced ions previously described for g it. will persist and the relays will not For the three phase thro luv L x capacitors take account of i. y I rent of the pilot wires. The same method of presentation is used o Figures 11 and 12. In this case the capaci ice of the hypothetical capac arc ea h equal to one fourth the capaacance to neutral of one pilot wire. Using the same basis as for the other two diagrams, that is, letting I equal one ampere and the resistors 24, 34, 38, 44, 24, 34, 36, and 44 each equal ohm, the potentials will be as indicated on the diagram. The capacitors C and C for each pilot wire are across points of zero poten l and therefore, supply no charging curr o the circuit. The capacitors C nd C are each between points of potent difference of one fourth volt and each supply .06 ampere to the circuit with the dire tion of flow as irdicated on Figure 13. These cl arging currents are vectorially 120 apart and add up to zero in the neutral. T he compensating capacitors at both ends of the circuit have each a potential across their terminals of one fourth volt, and hay '1 the same capacitance as each of the hypotln condensers supply .06 ampere to a circuit with the direction of How as indicated on the iagram. As these charging currents at both ends of the circuit are also vectorially 120 apart they add up to give complete cancellation of the pilot wire charging current. Therefore, a balanced condition will exist and the relays will not trip on through fault.

7 As previously stated, the contacts 23, 33 and 43 of the relays 22, 32 and 42 respectively control the circuit breaker or circuit breakers for disconnecting the end A of the line 1 from the system. In a system where individual circuit breakers are used for the respective line conductors 2, 3 and 4 the contact 23 controls the opening of the circuit breaker for the conductor 2, the contact 33 controls the circuit breaker for the conductor 3 and the contact 43 controls the circuit breaker for the conductor 4. In a systeni'wherein a three pole circuit breaker is used for opening all three of the line conductors 2, 3, and 4 simultaneously each of the contacts 23, 33 or 43 is arranged to control the circuit breaker and for this purpose these contacts are connected in parallel. The contacts 23, 33 and 43' control the disconnection of the end B of the line 1 from the rest of the circuit in a like manner. As previously stated, the construction of the pilot transformer 17 constitutes one of the features of my invention. The transformer is constructed. as has been previously pointed out. In addition while I have shown for illustrative purposes in the drawings the winding of the relay such as 22 connected directly to the end terminals of the secondary winding 21, the pilot transformer secondary winding 21 may actually be provided with several taps in order that a Wide range of current may be used on the overload relays such as 22 to suit different conditions.

Referrence may now be had to Figure 14 showing a somewhat different form of pilot circuit employing my novel pilot transformers. This diagram shows the necessary connections to protect a line against failures from one conductor to ground only. The same relay current transfm-mersmay be used in combination of three at each end of the line to protect against failures of any kind as pointed out above in connection with Figure 1', when this protection is thought desirable. In this figure similar parts to those shown in Figure 1 have been indicated by similar reference numerals. In this arrangement the current transformers 5, 6 and 7 are connected together and to the ground conductor 210 through the measuring instruments 8, 9 and 10. The current transformer 211 corresponds to the current transformer 11,

12 or 13 of Figure l. The pilot transformer 17 in this figure is identical to the pilot transformer 17 in Figure 1, the windings 16-20 and 16-19 having the same number of turns and wound upon a core in a manner which will produce close mutual magnetic coupling as in the previous transformer. The resistance 240 is of a value equal to the resistance of one of the pilot wire conductors or 59.

The condenser 250 is of a capacity equal to one half of the capacity bet-weenthe pilot conductors 55 and 59. The relay 22 through its contact 23 actuates the usual instrumentalities for opening the circuit through the three line conducors 2, 3 and 4 at the end A of the line and the relay 22 is effective through its contacts 23 to likewise open the circuit through the three line conductors of 

